Method of starting alternating-current motors



NOV-

I W. E. PAUL IETHDD 0F STARTING ALTERNATING CURRENT MOTORS Filed May 23. 1922 Inventor William 13. Paul, by 4D: His Attowvwe g.

Patented Nov. 10, 1925.

UNITED srA'rsgrArENr orrics.

WILLIAM E. PAUL, OF S CHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY. A CORPORATION OF NEW YORK.

METHOD OF STARTING A'IiTERNATING-GURRENT MOTORS.

Application filed May 23, 1922. Serial No. 563,154.

f0 all whom it may concern:

Be it known that I, VILLIAM E. PAUL, a citizen of the United States, residing at Schenectady, in the county of Schenectady,

' State of New York, have invented certain new and useful Improvements in Methods of Starting AlternatingCurrent Motors, of which the following is a specification.

My invention relates to the starting of alternating current motors by means of an auto-transformer, and it has for itsobject the improvement in the method whereby the motonis first connected to the source of supply through a reduced voltage connection of the transformer finally connected directly to the source without opening up the motor circuitduring the starting operations.

One of the objects of the invention isto reduce or entirely eliminate the transient currents of comparatively large value which occur in the ordinary methods of starting alternating current motors.

' More specifically, one of the objects the invention is to provide a method whereby the potential applied to the motor in starting is gradually increased from the initial low potential determined by the starting tap of theauto-tran'sformer to the full line potential Without either opening the motor circuit or dropping the applied po tential during the starting operations.

In certain of itsas-pects, the present invention is an improvement of the method described and broadly claimed in the patent to Max Korndorfer, No. 1,096,922, dated May 19, 1,914. In the Korndorfer method, the motor is connected to the source of supply through a reduced voltage tap of the auto-transformer, the neutral of the trans former is then opened so that the transformer acts as a reactor coil and finally the motor is connected directly to the source of supply without opening the motorcircuit during the starting operations: \Vhile the particular method selected by Korndorfer to illustrate his broad idea is entirely operative and practical for the ordinary conditions encountered, I have found that under certain circumstances, as, for instance, where a large alternating current motor is to be started, the potential applied to the motor may drop when the neutral of the transformer is opened and the transformer is used as a reactor coil, thus causing a transient current of a greater value than the load current of the motor-"When the motor is connected directly to the source of supply.

One of the objects of this invention is to prevent this drop in potential, and to either reduce or eliminate the transient current.

In carrying the invention into effect in one form, I cause the impedance of the auto-transformer serving as a reactor in the Korndorfer method to be reduced preparatory to connecting the motor directlv to the line. This maybe done by loading the auto-transformer by connecting an admittance such as a resistance about a portion of the windings after the motor is connected to the line through the reduced potential.

tap of the transformer, so that when the neutral of the transformer is opened, the impedance of the transformer when used as a reactor coil will be reduced and the otential applied to the motor will not drop. The admittance, such as resistance may be such that when the'neutral of the transformer is opened the potential applied to the motor will increase. Although the invention is particularly useful in starting large alternating current motors, it .is not necessarily limited thereto, since it is applicable generally in the starting of motors by means of an auto-transformer.

For a better understanding of the invention, reference is had to the accompanying drawing which illustrates a system of motor control intended to be operated in accord ance with the invention.

Referring to the drawings, the electric 7 motor 10 of the three-phase type is adapted to be connected to the three-phase source of supply 11 through the starting auto-transformer 12 for the purpose of supplyinga reduced potential to the motor preparatory to finally connecting the motor to the source of supply forfull speed operation. Theswitch 13 is provided for controlling the neutral point of the transformer and the switches 14, 15 and 16 are provided for controlling the circuit through the resistors 17, 18 and 19 respectively. The switch 20 is provided for connecting the motor directly to the source of supply.

\Vith the arrangement of parts as shown in the drawing, the method of starting the motor from the source of supply in accordance with the invention is as followsr Thc switch, 13 is thrown so as to close the new tral point of the transformer and the circuit breaker 21 is then closed so as to connect the motor to the source of supply through the reduced voltage taps of the starting transformer. After the motor has come up to the proper speed determined by the reduced potential applied to the motor through the starting transformer, the switches 14, 15 and 16 are closed so as to connect the resistors 17 18 and 19 across the low potential sections of the starting trans former. This has the effect of loading the transformer and thus reducing the impedance thereof when the neutral is subsequently opened and the transformer acts as a reactor. The neutral disconnecting switch 13 is then opened, thereby connecting the starting auto-transformer as a reactor coil between the source of supply and the motor. It will be seen that the sections 22, 23 and 24 are connected in series in their respective supply conductors of the connections for the motor. Because of the fact that the resistors 17 18 and 19 are connected about a portion of the compensator windings, the potential applied to the motor will not drop when the neutral disconnecting switch 13 is opened. That is because of the fact that the resistors constitute in effect a shunt for the auto-transformer which is now actin as a reactor and thus effects a reduction o the impedance thereof from that which the coils of the transformer would have if it were not for the resistors. \Vhen.

the motor has come up to the proper speed determined by this connection of the compensator, the line switch 20 may be closed, thereby connecting the motor directly to the source of supply and short circuiting the upper sections 22, 23 and 24 of the starting compensator. current will flow when the motor is connected directl to the source of supply, be-

cause of the act that the flux in the transformer core is a very low value. The motor is thereby started without either opening up the motor circuit during the starting operations or permitting the applied potential to the motor to drop during the starting operations. I

The resistors 17, 18 and 19 may be adjusted if desired so asto obtain the proper applied potential to the motor when the neutral of the compensator, is opened.

These'resistors make it possible to adjust the reactance of the'trans'former whenv the transformer 1s used as a reactor coil, so that the transformer may be designed for its principal purpose as a starting transformer to supply a reduced otential to the motor for the ori inalstarting connection.-

While -I ave shown the invention as applied to the starting of, a'three- )hase alter.-

nating current motor, I'would lave it un derstood that the inv'entionislikewisei ap-,

Practically no transient circuit.

of the starting transformer are loaded 50 as to reduce the impedance thereof, the method of the invention may also be carried into effect by loading the upper or higher voltage sections of the transformer,'or by loading any desired section or sect-ions of the windings. Furthermore, the transformer could be provided with an extra winding or windings inductively related to the ordinary windings, and the flux reduced in the transformer core by loading the extra windings- It will also be apparent to those skilled'in the art that in case it is desired to do so, the transformer may be completely disconnected from the source of supply after the line switch 20 has been closed to conniact the motor directly to the source of sup-- P y- The transientcurrent which flows when an alternating current motor is started in the ordinary way is objectionable because of the fact that there is apt to be a distor tion and a strain of the coils of the motor, thereby imposing limitations in the practical sizes of this type of motor. This feature is of particular importance in large alternating current motors, and although the feature is present when comparatively small alternating current motors are started by the ordinary starting transformer arrangement, the feature is not so objectionable in the smaller sizes of motors.

Due to the high saturation of the transformer core during the period when the coils are connected to function as series rcactors, three phenomena are observed as fol-- lows 1) A distortion of the wave shape due to a large fifth harmonic in the transformer exciting current. Under certain conditions this may cause disturbances inthe supply (2) Excessive voltage rise on the open ends of the transformer coils at the neutral point. This may result in insulation breakdown or arcing over in the transformer.

(33) An undesirable hum or noise due to the vibration of the transformer core laminations under the execessive saturation. This vibration may result in rapid wear and distortion of the core insulation.

My method practically eliminates all three of the foregoing disadvantages. It retains the advantage of not opening up the motor circuit during the transition from tap to full ling yoltage, and in addition it reduces the impedance of the transformer circuit so that the motor current is not choked down but fed steadily, thus keeping up the torque and the excitation on the mo tor. In this way, the motor Voltage may be brought nearly to normal, thus causing no disturbance when the main line switch is thrown to connect the motor directly to the supply circuit. In other words, a motor will coine up to speed with. a given load with less maximumcurrent taken from the line by my method than by any of the methods of the prior art. By keeping the flux in the transformer or reactor core of a low value during the transition from tap to full line voltage, in accordance with my method, and at the same time keeping nearly full excitation on the motor, the transient current on the motor when the main line switch is closed is practically eliminated, thus effecting a continuous uniform starting cycle withoutstrain or jar on the motor and with no abnormal disturbance of the supply circuit. 'It will be understood that the various switching operations may be effected automatically in response to a condition of the motor, such for example as the current taken by the motor, the motor speed, or in response to a time element device whiclncontrols the automatic closing of the switches in the proper sequence.

It should be further understood that while I have indicated resistors on the drawing, any suitable admittance such as. reactors may alsobe used for short eircuiting the auto-transformer windings, or a combination of resistors and reactors. Resistors are ordinarily cheaper than reactors, so that resistors will ordinarily be preferable, although the method of the invention is likewise applicable to other devices than resistors for keeping the flux in the core of the transformer low during the time the transformer serves as a reactor. It will be observed that the devices for loading the transformer, whether they be resistors or reactors, or a con'ibination of resistors and reactors, may be-designed for very short time service and hence may be very greatly overrated and built at a very small cost. This also applies to the resistor. switch or switches which control the devices for loading the transformer, because of the fact that they are required for only the transition period and not during the starting operations.

I would also have it undertood that while I have shown an arrangement in which the transformer is loaded after the motor is connected through the transformer to supply a reduced potential for starting, the invention may also be practiced by applying a very small load to the transformer initially and increasing the load after the motor has come up to speed attran'sformer tap voltage, thus the switches 14, 15, and 16 may be closed initially and the resistors 17,18

and 19 be adjusted so that they will have practically no effect untilafter the neutral is opened. After the motor has come up to speed on tap voltage, the resistors 17 18 and 19 may be gradually cut out of circuit preby Letters Patent of the United States, is

1. The method of starting an alternating current motor from a source of supply, which comprises.interposing an auto-transformer between the source of supply and the motor, controlling the transformer to reduce the impedance thereof when the transformer is caused to subsequently act as a reactor, then causing the transformer to act as a relatively low impedance reactor, and finally connecting the motor directly to the source of supply without opening the motor circuit during the transition.

. 2. The method of starting an alternating current motor from a source of supply, which comprises interposing an auto-transformer between the source of supply and the motor, then loading the transformer while connected in the motor circuit to reduce the impedance of the transformer when the transformer is subsequently caused to act as a reactor, then causing the transformer to act as a loaded reactance in the motor circuit and then connecting the motor directly to the source of supply without opening the motor circuit during the starting of the motor.

3. The method of starting an alternating current motor from a source of supply which comprises inter-posing an auto-transformer between the source of supply and the motor and then connecting the auto-transformer as a reactive coil with a local circuit established about at least a portion of the windings of the transformer.

4. The method of starting an alternating current motor which comprises successivelv connecting the motor through a reduced voltage connection of a transformer, establishing a local circuit about a portion of the transformer windings while the motor is connected to a source of supply through the transformer, then'eausing the compensator to act as a reactor with the said local circuit about the transformer windings maintained,"-

and then connecting the motor directly to the source of supply without opening the motor circuit.

5. The method of starting an alternating current motor which comprises successively connecting the motor to a source of supplythrough a reduced voltage connection of a transformer shunting a portion of the transformer windings, leaving the remain- 6. The method of starting a multiphase alternating current motor by means of an auto-transformer which comprises connecting the motor to a source of supply through a reduced voltage connection of the trans former, loading the transformer while connected in the motor circuit to reduce the impedance of the transformer after the neutral of the transformer is subsequently opened, opening the neutral point of the transformer while maintaining the loading of the transformer then serving as a relatively low impedance reactor, and then connecting the motor directly to the source of supply without opening the motor circuit.

7. The method of starting a multiphase alternating current motor which comprises successively connecting the motor to a source of supply through a reduced voltage connection of an auto-transformer, connecting resistance about the portion of the windings of the auto-transformer between the neutral and the reduced voltage connection for the motor, opening the neutral of the auto-transformer while maintaining the connection of the-said resistance to the said windings, and then short circuiting the portion of the auto-transformer windings remaining in the motor circuit.

' In witness whereof, I have hereunto set myhand this 22nd day of May, 1922.

WILLIAM n PAUL. 

